The present invention relates to a novel apparatus for pumping two separate flowable materials at a constant rate of flow according to a predetermined changing ratio or a gradient. The present invention is particularly useful for, although not limited to, delivering a gradient of two solvents to a liquid chromatographic column to accomplish the analysis of a liquid sample.
More specifically, a liquid chromatographic column contains packing of finely divided particles for solute separation of the liquid sample. Typically, the particulate size of the packing is about 5 microns. To overcome the resistance to flow established in a packed column requires a pulse free, high pressure pump.
Another problem encountered is the different solubility characteristics of the solutes of a liquid sample. The object of efficiency in analyzing the liquid sample dictates that two solvents be used to elute characteristically different solutes of a liquid sample through the packed column. For example, a polar solvent would wash a polar liquid solute through the packed column at a satisfactory rate, while nonpolar solutes would be carried at an undesirably slow rate. Therefore, a second non-polar solvent is pumped at predetermined gradient with respect to the polar solvent; a technique known in the art as "gradient elution analysis". This technique allows the polar and non-polar solutes to be transported through the packed column within a reasonable time span.
Presently, a typical gradient delivery system uses two pumps, each having a motor and a means to program the flow rate of each pump's output. This method entails the difficult coordination of two power sources. The apparatus for such a device tends to be bulky and difficult to fit into a laboratory space.